A hydraulic system for actuating at least one shift element of a transmission is known from DE 10 2011 079 850 A1. The hydraulic system is formed with multiple valve devices and a hydraulically actuated parking brake unit. One of the valve devices is a parking brake valve with multiple valve seat pockets formed in the area of a valve housing, through which an actuating pressure of the parking brake unit that exists as a function of a supply pressure can be applied to it. The valve seat pockets can be brought into operative connection with each other or are separable from each other by a valve slide that is longitudinally adjustable in the valve housing. The valve slide is spring-loaded in the direction of a first axial position corresponding with a first operating state of the parking brake valve, in which actuating pressure is applied at the parking brake unit, at which the parking brake is transferred into the engaged operating state. In the area of a control surface of the valve slide, a pressure signal acting in the direction of a second axial position of the valve slide corresponding with a second operating state of the parking brake valve is connectable, in which, at the parking brake unit, an actuating pressure transferring a parking brake of the parking brake unit into the disengaged operating state can be applied.
The pressure signal that can be applied to the control surface of the valve slide is adjusted in the area of an electrically actuated pressure regulating device, whereas the pressure regulator features a so-called “rising characteristic.” This means that, with an increasing actuating current, the pressure outputted in the area of the pressure control device or the value of the pressure signal that can be applied in the area of the control surface increases. With a loss of the power supply of the hydraulic system, this design of the known hydraulic system leads to the fact that the pressure signal drops to the value of zero and the valve slide of the parking brake valve is transferred into the first axial position and the parking brake attempts to move into the engaged operating state.
Above a defined rotational speed of a parking interlock gear, in which a parking pawl then attempts to come in, the coming in of the parking pawl is avoided by a corresponding structural design of the parking interlock gear and the parking pawl, which are each designed with a correspondingly repelling contour. However, above the defined rotational speed, this structural configuration that avoids the coming in of the parking brake leads to so-called “ratchets” between the parking pawl not coming into the parking interlock gear and the parking interlock gear, whereas the parking pawl and/or the parking interlock gear suffer irreversible damages when there are long-persisting ratchets.
In order to prevent damages in the area of the parking brake unit, the hydraulic system is designed with a so-called “emergency gear concept,” which is automatically activated upon a loss of the power supply. During the activated hydraulic emergency run concept, in an automatic transmission featuring the hydraulic system, depending on the operating state, so-called “emergency gear transmission ratios” are engaged, for the presentation of which one of the two shift elements is switched on or closed by applying a corresponding actuating pressure. The respective lines subjecting the shift elements to actuating pressure are in operative connection with each other through a so-called “ball shuttle valve.” Through the ball shuttle valve, it is ensured that, in each case, the higher actuating pressure of the two shift elements provided for the presentation of the emergency gear is forwarded in the direction of the parking brake valve and is applied there in the area of a further control surface. From the actuating pressure, applied at the further control surface of the valve slide of the parking brake valve, of one of the two shift elements switched on for the presentation of the emergency gear, an actuating force acting on the valve slide results, which transfers the valve slide into its second axial position or hold it in this, in order to hold the parking brake in the disengaged operating state.
The emergency gear functionality described above of the known hydraulic system is characterized by a high design effort in the area of the hydraulic shift device and therefore causes high production costs.